It can be a fee per show or per ticket....and you can itemize the details at the checkout confirmation.
The actual fee must be set in the Admin>Products>Edit Category and the 'Order Total' module must be enabled from Admin>Payments>Order Totals with other fees.
As usual you must arrange the order of the modules very carefully. Either delete and install all again or arrange the data in Module Options e.g ot_service_fee.php;ot_advance_fee.php;ot_subtotal.php;ot_total.php
You'll get something like this at the checkout confirmation order totals if there are multiple Advance Fees.
The text can also be edited in the associated language file:
includes/languages/modules/order_total/ot_advance_fee.php
define('MODULE_ORDER_TOTAL_ADVANCE_FEE_TITLE', 'Advance Fee');